1. Field of the Invention
The present invention relates to a retractor for a seat belt device, and more particularly to a seat belt retractor having an energy absorbing mechanism including a torsion bar.
2. Description of the Related Art
A retractor for seat belt of a motor vehicle is used for restraining a passenger on a seat of the vehicle to secure a safety of the passenger. In this field of the seat belt retractor, an emergency-locking type belt retractor is used which includes an emergency locking mechanism for physically locking a retractor by means of an inertia sensing means responding to abrupt acceleration, impact, or abrupt deceleration, whereby it effectively and safely restrains the passenger.
U.S. Pat. Nos. 3,929,300 and 4,366,934, and Japanese Utility Model Publication No. Hei. 2-45088 may be enumerated for the conventional techniques for the emergency-locking type belt retractor. In the locking mechanism disclosed in the above publications, an engaging member which is provided at one end of a take-up shaft having a webbing wound thereon, in case of emergency, comes in engagement with an engaging-member receiving part of a retractor base, to thereby lock the rotation of the take-up shaft in the web unwinding direction.
In a situation where a collision occurs, and the emergency locking mechanism of the emergency-locking type belt retractor operates in response to the collision to lock the webbing in its pulling-out motion, if an impact force by the collision is extremely large, a webbing tension increases with the elapsing of time after the collision. As a result, an abrupt deceleration is generated in the passenger body, and a load acting on the passenger from the webbing is extremely large. Various proposals to cope with this problem have been made. One of those proposals uses an energy absorbing mechanism which absorbs an impact force acting on the passenger body in such a manner that when a load acting on the webbing exceeds a predetermined value of load, it unwinds the webbing with a predetermined length.
Many seat belt retractors with the energy absorbing mechanisms have been proposed (e.g., U.S. Pat. Nos. 5,344,095 and 5,526,996).
This type of the seat belt retractor comprises:
a take-up shaft, substantially cylindrical, on which a webbing is wound; PA1 a torsion bar passing through a center hole of the take-up shaft, one end of the torsion bar being coupled with the take-up shaft in an unrotative fashion while the other end thereof being coupled with a locking base also in an unrotative fashion; and PA1 emergency locking means causing, in case of emergency, the locking base to engage with a retractor base to lock the torsion bar in its rotation and hence to lock the rotation of the take-up shaft in the direction in which the webbing is pulled out; PA1 wherein when a tension, which acts on the webbing during the operation of the emergency locking means, exceeds a predetermined value of tension, an impact energy is absorbed through a torsional deformation of the torsion bar while at the same time a tension acting on the webbing is controlled.
Thus, the above impact energy absorbing technique absorbs the impact energy through a deformation of the torsion bar passing through the center hole of the take-up shaft. Another impact energy absorbing technique is known which absorbs the impact energy through a plastic deformation of the take-up shaft per se, the retractor base or the like. When comparing with the latter technique, the former technique has the following advantages: 1) A design freedom is high in the design on an energy absorbing amount and the unwinding amount of the webbing, and 2) the structure is simple and hence advantageous in size reduction of the belt retractor.
The number of twists of the torsion bar in the conventional seat belt retractor is inversely proportional to the diameter of the torsion bar; viz., it increases with decrease of the bar diameter or decreases with increase of the bar diameter. On the other hand, the number of twists of the torsion bar in the conventional seat belt retractor is proportional to the length of the torsion bar.
Therefore, attempt to obtain a high torsion torque by increasing the bar diameter entails an insufficient number of twists of the torsion bar. Where a long energy absorption length is required, the torsion bar needs to be elongated. However, attempt to simply elongate the torsion bar is rejected since there is a limit in increasing the size of the seat belt retractor when it is mounted on the vehicle body, and the retractor base has a predetermined width.
Further, as described above, the torsion bar is known of which one end or first end is coupled with the take-up shaft in an unrotative fashion and the other end or second end is coupled with a locking base also in an unrotative fashion. This torsion bar sometimes changes its axial length through a twist deformation of the torsion bar when the energy absorbing progresses.
When the axial length of the torsion bar increases with progress of the torsion deformation, the locking base that is coupled with the second end of the torsion bar moves outside beyond the retractor base. As a result, the axial alignment in the emergency locking means, which engages the locking base with the retractor base in case of emergency, is lost, and its locking strength is possibly reduced.
In a case where the stopper means, which stops the rotation of the take-up shaft in the unwinding direction of the webbing when a torsion quantity of the torsion bar exceeds a predetermined quantity of torsion, includes 1) a pair of guide grooves, shaped like C, which are formed in the end face of the locking base closer to the take-up shaft and the end of the take-up shaft closer to the locking base, and 2) a lock piece slidable along the inner walls of those grooves, if the axial length of the torsion bar increases with process of the torsion deformation, an interstice between the end face of the locking base closer to the take-up shaft and the end face of the take-up shaft closer to the locking base is widened. The axial coupling portion of the lock piece with the grooves is reduced. The necessity to cope with this is to deepen the grooves and to increase the size of the lock piece.